The present invention relates to an ester of iodohippuric acid, and more particularly to a radiolabeled ester of iodohippuric acid, a stable conjugate thereof with a protein and a method of making the same.
Radioiodine labeled proteins of high specific radioactivities have wide application in a variety of biochemical studies. Specific antibodies labeled with various radionuclides are increasingly being applied for visualizing undetectable tumors and metastases. In addition, labeled antibodies offer a means of radioimmunotherapeutic ablation of tumors for which conventional therapies are ineffective.
A variety of procedures for labeling large molecules (e.g., macromolecules such as proteins) with radioiodine have been developed over the years. The most widely used chloramine-T procedure offers an efficient method for the substitution of iodine into the tyrosine residues of protein molecules and antibodies. However, some disadvantages of this procedure are: (i) polypeptides that lack tyrosine cannot be iodinated; (ii) oxidizing and reducing agents denature the protein to a certain extent. The rapid metabolism of radioiodinated antibodies leads to the incorporation of metabolized radioiodine into the thyroid and high levels of circulating radioiodine in the blood mask tumor uptake considerably.
The radioiodine-labeled iodohippuric acid, and in particular the (.sup.125 I)-orthoiodohippuric acid (OIH), would be a suitable radiolabel if it could be conjugated with proteins and antibodies under reaction conditions which would not adversely affect the components of the protein. The (.sup.125 I)-OIH-labeled protein offers the advantage of rapid blood clearance of the radioiodine labeled hippuran moiety upon metabolic breakdown of the conjugate so that the target-to-background ratio would be enhanced at short time intervals after injection with the conjugate and the risk to patient health from radioactivity minimized.
Accordingly, it is an object of the present invention to provide a novel ester of iodohippuric acid when can be radiolabeled.
Another object is to provide a radiolabeled ester of iodohippuric acid which is capable of forming a stable conjugate with a macromolecule such as a protein, especially under mild conditions.
A further object is to provide a protein compound useful as a radiolabel comprising a stable conjugate of a radiolabeled ester of iodohippuric acid and a protein, which conjugate upon metabolic breakdown provides rapid clearance of the radiolabeled hippuran moiety.